Electroplating apparatus with plating vessel having protrusions and recesses

ABSTRACT

AN ELECTROPLATING APPARATUS WHEREIN ARTICLES TO BE PLATED ARE PLACED IN A VESSEL AND IMMERSED IN A FUSED SALT PLASTING BATH. THE VESSEL IS THE CATHODE AND AN ANNULAR ANODE IS DISPOSED OVER THE ARTICLES. THE VESSEL IS INTERMITTENTLY ROTATED DURING THE PLATING OPERATION TO DRIVE THE ARTICLES TOWARDS THE PERIPHERY AND THEN GATHER THEM IN THE CENTER OF THE VESSEL. THE VESSEL IS PAN-SHAPED AND HAS PROTRUSIONS AND RECESSES IN THE SURFACE THEREOF TO PROVIDE BETWEEN CONTACT WITH ARTICLES TO BE PLATED, AND HAS PERFORATIONS THEREIN TO ENABLE THE FUSED SALT BATH TO FLOW THERETHROUGH.

May 7, 1974 AKIRA MIYATA ETAL 3,809,641

ELECTROPLATING APPARATUS WITH PLATING VESSEL HAVING PROTRUSIONS ANDRECESSES Filed Sept.

3 Sheets-Sheet 2 May 7, 1974 AKlRA MlYATA ETAL 3,809,641

' ELECTROPLATING APPARATUS WITH PLATING VESSEL HAVING PROTRUSIONS -ANDRECESSES Filed Sept. 8, 1972 5 sheets sheet 5 Unitecl States Patent3,809,641 ELECTROPLATING APPARATUS WITH PLATING VESSEL HAVINGPROTRUSIONS AND RECESSES Akira Miyata, Tokyo, Hideyo Okubo andChikayoshi Tomita, Yokohama, and Akio Suzuki, Tokyo, Japan, gssignors toNippon Kokan Kabushiki Kaisha, Tokyo,

apan Continuation-impart of application Ser. No. 57,865, June 12, 1970,which is a division of application Ser. No. 747,367, July 24, 1968, bothnow abandoned. This application Sept. 8, 1972, Ser. No.'287,271

Claims priority, application Japan, May 11, 1968, 43/31,518 Int. Cl.C23b 5/78 U.S. Cl. 204-213 18 Claims ABSTRACT OF THE DISCLOSURE Anelectroplating apparatus wherein articles to be plated are placed in avessel and immersed in a fused salt plating bath. The vessel is thecathode and an annular anode is disposed over the articles. The vesselis intermittently rotated during the plating operation to drive thearticles towards the periphery and then gather them in the center of thevessel. The vessel is pan-shaped and has protrusions and recesses in thesurface thereof to provide better contact with articles to be plated,and has perforations therein to enable the fused salt bath to flowtherethrough.

This is a continuation-in-part of U.S. patent application Ser. No.57,865, filed June 12, 1970 and now abandoned, which is a division ofU.S. patent application Ser. No. 747,367, filed July 24, 1968, nowabandoned.

This invention relates to a plating apparatus, and more particularly toan apparatus for plating metals on a plurality of small articlessimultaneously and uniformly in a fused salt plating bath.

In the electroplating of bolts, washers, nuts, fasteners or speciallyshaped small articles for architecture or other uses, it is oftenrequired to plate a plurality of such articles simultaneously anduniformly. A prior method, called barrel plating, comprises placing alarge quantity of articles to be plated in a hexagonal or octagonalboxshaped cage and electroplating these articles with a water solutionwhile rotating this cage. However, since a large quantity of articlesconfusingly placed in a cage-shaped barrel are being electroplated whilesaid barrel is being rotated, such prior art apparatus does not permitsatisfactory plating of articles in the middle of the stack of articles.Articles contact one another loosely during rotation and the higherelectric resistance at least in a portion of the article, in contactwith another article in a moving state does not permit the flow ofenough cathodic current uniformly all over the accommodated articles.The articles plated are thus limited to these in the vicinity of theoutside of the stack of articles. Articles in the middle or bottomportion of the stack are not properly plated, because they are apt tobecome an anode relative to those at the outside. Articles can beproperly plated only when they are exposed alternately to the surface ofthe stack by being turned over. This hampers obtaining uniform plating.

Further, such prior art apparatus is limited to water solutionelectroplating. If such prior art barrel plating is applied to fusedsalt electroplating, the strong corrosive action of the fused salt willdissolve ground metal. Articles, for example, in the middle or bottomportion of the stack are put in the anodic state by the plating current,or previously plated metal. These phenomena degrade the quality ofplating or make articles incapable of being plated.

ice

On the other hand since it is practically impossible to treat many smallarticles one by one by ordinary elec troplating, aluminum plating cannotbe applied to small articles though this prior plating method has beenknown to have excellent properties such as high oxidation resistance.

Prior art devices utilizing the aqueous solution barrel plating methodhave been disclosed, for example, in U.S. Pat. No. 1,899,679; U.S. Pat.No. 843,321; and U.S. Pat. No. 951,662.

These prior systems have a common feature that the plating solution isagitated violently. In fused salt electroplating, however, if the fusedsalt bath is agitated violently, the surface thereof absorbs moisture inthe air and causes decomposition of the bath, bringing about undesirableresults.

Where an electrolytic cell having a circular configuration in the bottomand sides is used, as in the case of West German Pat. No. 857,880,frequent collisions occur among the articles to be plated during theirmovement. Such a cell cannot be used in fused salt electroplating.

The present invention is intended to improve the above prior art ofelectroplating, and the primary object of the present invention is toprovide apparatus having a simple structure wherein a uniformdistribution of cathodic current is applied to all the articles to beplated, when electroplating a considerable number of small articles, soas to make electroplating effective at all times.

A further object is to provide a plating apparatus particularly suitablefor use in a fused salt plating bath, and wherein part of theelectrolyte in the vicinity of the articles is periodically moved.

SUMMARY OF THE INVENTION The present invention adopts a specialpan-shaped cathodic vessel in which a predetermined quantity of smallarticles are accommodated. The vessel is adapted to be intermittentlyrotated at a circumferential speed which is suflicient to move thearticles outwardly by centrifugal force. Thus, due to the revolution ofthe vessel, the articles are spread toward the circumferential edge ofthe vessel. When the vessel is at rest, the articles then roll downtoward the middle portion of vessel, to form a thin layer, preferablyonly one layer, of articles to be plated. Thus, a constantly uniformcurrent distribution is obtained for all small articles in the vessel,and the tendency that some portion will receive an insufficient flow ofplating current because of stacks of articles, as occurs in the priorart, is substantially prevented.

Further, the present invention provides improved panshaped vesselstructures which enable better contact between the vessel and thearticles to be plated. The vessel has an irregular surfaceconfiguration, with protrusions, recesses, or the like, which provideimproved contact and lower plating resistances.

As heretofore mentioned, due to strong corrosive action of fused saltelectroplating, ground metal to be plated and previously plated metalare inevitably dissolved in case of the prior art barrel platingtechnique. This causes the degradation of the plating property, or evennonplating. According to the present invention, however, thesubstantially uniform distribution of current to all of the articlesaccommodated in the cathodic vessel effectively prevents the dissolutionof the metal articles. The resulting products show good quality plating.

With the present invention, aluminum electroplating can beadvantageously applied to small articles, thereby providing productshaving outstanding chemical and heat resistance. The prior art aluminumelectroplating cannot be done effectively to small articles so thatalmost all aluminum plated products have not been commercialized.

The present invention can apply fused salt electroplating effectively inthe above stated way without dissolution of ground metal and platedmetal.

It is advantageous to provide aluminum electroplated fasteners or othersmall products for use in, for example, aircraft, high speed vehicles,etc. This is advantageous since such small articles would causecorrosion at the metal parts if they were not aluminized articles. Thepresent invention can provide such products which were in factunobtainable with the prior art plating apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of aplating apparatus of the present invention;

FIGS. 2 and 3 partially show the states of arrangement of small articlesin a vessel, FIG. 2 showing the state when the vessel is standing stilland FIG. 3 showing the state when the vessel is rotating;

FIGS. 4A and 4B illustrate vessel configurations in accordance with thepresent invention;

FIGS. 5, 6A, 6B and 6C illustrate other vessel configurations inaccordance with the present invention; and

FIGS. 7 and 8 illustrate the contact of a bolt to be plated with a priorart vessel and with a vessel of the present invention which isillustrated in FIG. 8, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 generally shows anapparatus according to the present invention. A generally pan-shapedvessel 2 with a relatively shallow and gently arcuately concave bottomand open in the upper portion is mounted in a tank 1 containing platingbath 4. Vessel 2 is only generally shown in FIG. 1. Detailedillustrations of improved vessels 2 in accordance with the presentinvention are shown, for example, in FIGS. 4-6. Connected to the centerportion of vessel 2 is rotary shaft 3 which is secured to vessel 2 by anut 32 and the flange seat 31 provided on the rotary shaft 3. Cathodiccurrent is supplied via shaft 3. Panshaped vessel 2 is curved such thatarticles 25 contained in the interior of the vessel gather automaticallyat the center portion under the action of gravity and moreover arerolled or slid by centrifugal force toward circumferential edge ofvessel 2 when vessel 2 is rotated at a given speed. On the opening edge23 of vessel 2 is an annular cover 22 for checking the articles fromspilling over the outer edge of vessel 2 when the vessel is rotated.

Vessel 2 is made of aluminum and is provided with perforations 21 overthe surface thereof for in-flow and outflow of a fused salt baththerethrough. Vessel 2 is coated on the backside with chemical and heatresistant synthetic resins such as polytetrafluoroethylene. The cover 22is also formed of polytetrafluoroethylene or other material. An annularaluminum anode 10 is concentrically mounted about the rotary shaft 3over the vessel opening 24. The anode 10 is supported in place bysupport member 11 and supplied with anodic current. Tank 1 is providedat least on its inside surface with an anticorrosive layer such as aglass lining and on the top with a first cover 12 made ofphenol-formaldehyde resin or polytetrafluoroethylene. From the openingportion of cover 12, a cylinder 6 made of polytetrafluoroethylene orother anticorrosive material, having an inner diameter somewhat largerthan the outer diameter of pan-shaped vessel 2 is suspended in the fusedsalt bath 4. The portion of cylinder 6 which is immersed in the fusedsalt bath is provided with perforations 17 for passing said baththerethrough. The top opening portion of cylinder 6 is provided with asecond lid 7 also made of phenolformaldehyde resin orpolytetrafluoroethylene. Through opening 7' provided centrally in lid 7are inserted the aforementioned rotary Shaft .3 and pp member asillustrated in FIG. 1.

On one side of the second lid 7 is provided another opening 8 forinserting articles which are to be plated. Around the opening 8, it ispreferable to form a gas flow curtain by jetting an inactive gas so asto stop air from intruding into the fused salt bath. Further, there isan annular floating piece 14 made of glass or other anticorrosivematerial which is floated on fused salt bath 4 in cylinder 6 andprevents evaporation of bath. Further evaporation preventive material 13such as silicagel is also floated on the surface of bath 4 betweencylinder 6 and tank 1 so as to cover the surface of the electroplatingbath. At the bottom portion of tank 1 and also in other sections arearranged heaters 5 so as to keep the temperature of the bath aspredetermined in the interior of tank.

Rotary shaft 3 is designed to be given a predetermined rotary movementby a rotary mechanism such as a motor 35 shown generally in FIG. 1.Inactive gas inlets 15 and 16 are provided respectively on lids 12 and7. A feeder collar or ring 26 is provided for furnishing electriccurrent to rotary shaft 3.

FIGS. 4-6 illustrate a number of pan-shaped electroplating vesselsaccording to this invention which provide improved contact with thearticles to be plated, thereby resulting in improved plating uniformity.FIG. 4A is a vertical sectional view of a vessel 56 according to thisinvention and FIG. 4B is a top plan of the same vessel. The vessel 56has perforations 52 therethrough. Aluminum bars 51 arranged in aspiral-type configuration are fixed on the surface of vessel 56 by meansof fixing members 53. The aluminum bars 51 constitute protrudingportions of the vessel structure and the spaces therebetween constitutereceding portions of the vessel structure. Perforations 52 provide pathsfor the fused salt to pass through during operation. The bars 51 areshown in crosssection only by way of example, it being understood thatthey may take any other convenient shape. In a typical example, thevessel may have a diameter of 365 mm. and the round aluminum bars mayhave a diameter of 3 mm. Articles are plated more uniformly using thevessel of FIGS. 4A and 4B, as will become more apparent from thediscussions hereinbelow with reference to FIGS. 8A and 8B.

FIG. 5 shows a modification of FIGS. 4A and 4B wherein square aluminumbars 54 are used. In this case the square bars may be of a size of 2 In.x 5 mm., for example.

FIGS. 6A and 6B show sectional and top views, respectively, of a vessel58 provided with a plurality of generally hemispherical cavities 55having small perforations 52 over the surface thereof. The perforations52 are in the depressions or cavities 55, but may be otherwise located.One typical example of the size of the cavity is 10 mm. in diameter and5 mm. in depth, and that of the perforation is 3 mm. in diameter. InFIG. 6, reference numeral 59 denotes the protruding portions of thevessel and numeral 55 (cavity) denotes the receding portions of thevessel.

The above dimensions for FIGS. 4-6 are mere examples. They may varydepending on the types of articles to be plated and the conditions oftreatment, as should now be apparent.

Various other vessel configurations may be used-the critical featurebeing the provision of protruding and receding portions in the surfaceof the vessel which contacts the articles to be plated.

In the present invention, a shallow, gently curved panshaped vesselhaving protrusions and recesses is used so that a wide contact area isprovided between the vessel and articles to be plated, such as bolts,thereby permitting electric current to pass more evenly between thevessel and the articles. As shown in FIG. 7, when a panshaped vessel 2'having a smooth surface is used for electroplating bolts 40 and thelike, the vessel 2 and the bolt 40 contact each other at substantiallyonly two points. As

seen from FIG. 7, the contacting points are few in number and narrow inarea. Therefore, application of an electric current causes high contactresistance between the vessel 2' and the article 40. As a result, (1)insuflicient plating current flows from the vessel 2 to the bolt 40; (2)the bolt 40 has an electrical potential nearer to that of the anode thanthe vessel 2 and the aluminum film earlier deposited on the surface ofthe bolt 40 tends to become dissolved in the fused salt again; and (3)excessively thick aluminum films tend to be deposited at contactingpoints between the vessel 2' and the bolt 40.

Thus, with the apparatus of FIG. 7, the outer diameter of the screwthread of bolt 40 tends to become too thick at the tip of the bolt whereit makes contact with cathodic vessel, resulting in the production ofdefective bolts.

In order to obviate these defects, the present invention adopts apan-shaped vessel as shown, for example, in FIGS. 4-6. FIGS. 8A and 8Billustrate the contact of a bolt 40 with vessels 57 and 58 of FIGS. 5and 6, respectively. As seen from FIGS. 8A and 8B, the contact area ofthe bolt 40 with the vessel surface is large, thereby permittingelectrical energy to flow more uniformly between the vessel and the bolt40.

In the present invention the articles to be placed have the portionsthereof to be plated periodically changed, with the result that uniformplating all over the article is effected during the whole plating time.When the panshaped vessel is rotated via the rotary shaft 3 afterturning off the current in the course of electrolytic operation, thearticles to be plated move toward the peripheral portions of the vesselunder the influence of centrifugal force. When all of the articlesfinish moving and are in the position shown in FIG. 3, the rotation ofthe rotary shaft 3 is stopped. The articles then roll down toward thecenter of the vessel and rest on the bottom with the projecting portionsof the articles located in the receding portions of the vessel surface,and the receding portions of the articles lying on the projectingportions of the vessel (see FIG. 2), the articles being so positioned asto receive plating on new portions. Then, direct current is againapplied between the anode and the vessel to continue electrolysis.Rotation of the rotary shaft 3 is repeated a plurality of times duringthe plating operation so as to have the entire surface of the articleuniformly plated.

FIG. 2 shows the arrangement of articles to be plated when the vesselstands still during plating. FIG. 3 shows their state when they arespread by centrifugal force caused by the rotation of the vessel. Thesmall articles 25 are shown in a collective state, but in a thin orsingle layer, in FIG. 2, and in a developed state in FIG. 3 just afterrotation of the rotary shaft 3. As shown in these figures, an area S orS quite free of articles is developed at outer peripheral portion or themiddle portion of vessel 2. The repetition of these states not onlyrotates small articles 25 in groups but also changes their positions.While small articles such as screws or nuts transfer from the state ofFIG. 2 to a developed state of FIG. 3, they roll in the vessel and aredispersed toward the periphery so as to be separated by a wider distancefrom each other. It is evident that the articles 25 are changed inposition and coordination in the developing and gathering process, andconsequently, uniform plating is obtained.

A feature of the present invention is that the panshaped vessels areprovided with small perforations all over the surface thereof so as toshake otf the fused salt in the vicinity of the articles (21 in FIG. 1;52 in FIGS. 4 and 5; and 52 in FIG. 6). In aluminum electro-plating bymeans of a fused salt bath, molecular aluminum salt (AlCl in the bath isreduced on the surface of the article and is deposited in the form ofaluminum (Al+++Al). Therefore, there must exist in the vicinity of thearticle sufficient aluminum salt for aluminum depositing. According tothe present invention wherein perforations are provided in the vessel,aluminum salt is newly supplied into the vessel through said perforationas the aluminum salt loses aluminum and is pushed toward the peripheralportions of the vessel by centrifugal force caued by the rotation of thevessel.

The present apparatus is also provided with a cylinder 6 in FIG. 1having perforations 17. This further facilitates circulation of thefused salt and also prevents the agitation action of the fused salt fromreaching the surface of the bath where it touches the air. As describedabove, the fused salt where it is contacted with the article is moved byagitation but this movement does not reach the surface of the bath whereit touches the air.

Still another feature of this invention is "that the ap paratus involvedis of a simple structure. Since a fused salt (a mixture of A1Cl NaCl,and KCl) has strong corrosiveness against the apparatus, the platingapparatus mus-t be of as simple construction as possible, so as toenable economical replacement of corroded parts thereof. The apparatusof the present invention clearly fulfills this object by utilizing apan-shaped vessel which is inexpensive to fabricate and which is easilyreplaced.

The pan-shaped vessels 2, 56, 57, 58 serve as the container of articles25 to be plated and at the same time as the cathode electrode. Thevessels 2, 56, 57, 58 can be made of any metal of high electricconductivity. For example, for effective fused salt electroplating notonly can a vessel made of aluminum as ground material be used, but alsoone made of a non-metal such as porcelain, glass lined with a copper netas a cathodic plate, or others can be used. The fact that the back sideof such a vessel forms an insulating surface prevents useless flow ofcurrent from the bottom. To prevent a useless flow of current fromrotary shaft 3 and support member 11, the portions thereof in contactwith the fused salt bath would preferably be protected 'with ananticorrosive coating such as polytetrafluoroethylene. Rotary shaft 3and support member 11 can be made of glass or porcelain hollow material,the interior of which can be used as a passage of cathodic current. Theannular anode is kept almost an equal distance from the surface of allarticles to be plated and may be designed to stand still above thevessel.

An example of the operation of the apparatus of the present inventionwill now be given with reference to the basic structure of FIG. 1 usingthe vessel 58 of FIGS. 6A and 6B.

The fused salt contained 60 mol percent of AlCl 25 mol percent of NaCl,and 15 mol percent of KCl. The temperature applied was 160 C.Electroplating was conducted by applying direct current between aluminumand the vessel, the aluminum former acting as the anode and the vesselacting as the cathode. Current was turned on to begin plating.

After a predetermined length of time, the current was turned ofi and thevessel 58 was rotated by turning the rotary shaft 3 by means ofenergizing the motor 35. Rotation of the vessel 58 was stopped after allof the articles had moved and returned, and direct current was againapplied between the anode and the cathode. These processes were repeatedduring the whole plating time. FIGS. 2 and 3 show the general positionof the articles before and just after rotating the vessel, respectively.

A movement cycle comprising a rest of 6.5 seconds and rotation of 0.5second was used. Current for plating was 6.5 v., a. Plating time was 40minutes. The entire lot of finished products proved to be satisfactorilyaluminum plated with a thickness of 15 microns.

Rotation of the rotary shaft 3 is conducted, for example, bycontinuously exciting the magnetic field of the DC shunt motor 35 andsending current intermittently to the armature circuit of the motor by amotor timer.

In carrying out plating, attention need be given to the following pointsin rotating the vessel: (1) acceleration in starting the rotation of thevessel; (2) decelerationin terminating the rotation of the vessel; (3)rotation speed of the vessel; (4) the number of revolutions of thevessel in one rotating period and the length of time involved; and thelength of time during which the vessel lies stationary in each cycle ofoperation. These points will be determined for each case, depending uponthe type of article being plated, the quantity, size of the platingapparatus, shape and configuration of plating vessel, etc.

When bolts were electroplated in accordance with the fused salt aluminumplating method by utilizing the apparatus shown in FIG. 1, with thesmooth surface vessel 2, inferior plated articles mounted to When thevessels 56, 57, 58 shown in FIGS. 4, 5 and 6, respectively, wereutilized in the apparatus of FIG. 1 in accordance with the presentinvention, the result was about 2 to 3% inferior products on theaverage.

It should be clear that various modification can be made to theillustrated embodiments within the spirit of the present invention asdefined in the accompanying claims.

What is claimed is:

1. Apparatus for electroplating articles comprising:

a tank for accommodating a plating bath;

a pan-shaped vessel having an upwardly curved, substantiallyhorizontally extending, bottom and a wide upper opening and adapted tobe immersed in said plating bath, the upward curvature of said vesselbeing upward from the center thereof toward the periphery thereof, theupwardly curved bottom of said vessel being adapted to retain articlesto be plated thereon in a layer during the plating operation of saidelectroplating apparatus, and said vessel constituting the cathode ofthe electroplating apparatus, said curved bottom of said vessel beingperforated and having an irregular surface configuration to provide morethan point contact between the vessel and the articles to be plated;

a cover portion provided at the periphery of said vessel and eXtendinginwardly and upwardly of said vessel at an acute angle with thehorizontal for preventing said articles which are being plated fromfalling out of said vessel during operation of said electroplatingapparatus;

a substantially cylindrical member surrounding at least a portion ofsaid pan-haped vessel in said bath, said cylindrical member having aplurality of perforations therethrough for inflow and outflow of saidplating bath;

means including a member extending substantially vertically into saidtank and connected to said panshaped vessel for supporting saidpan-shaped vessel completely immersed at a predetermined depth in saidtank, and for imparting intermittent rotation to said pan-shaped vessel,said rotation time being short relative to the time said vessel isstationary, so as to move and spread out said articles toward theperipheral portions of said vessel to form said layer of articles;

an anode member disposed over said pan-shaped vessel;

and

means for applying plating current to said anode and said cathode vesselwhen said vessel is stationary and for turning said plating current offwhen said vessel is rotating.

2. Apparatus according to claim 1 further comprising a heating means atthe bottom of said tank.

3. Apparatus according to claim 1 wherein said bottom of said pan-shapedvessel is shaped and dimensioned such that articles to be plated areformed into a single layer on the bottom surface of said vessel by saidintermittent rotation.

4. Apparatus according to claim 1 comprising a plurality of perforationsall over the surface of said pan-shaped vessel.

5. Apparatus according to claim 1 including a plurality of protrusionson the upper surface of the bottom of said pan-shaped vessel.

6. Apparatus according to claim 1 comprising a floating member adaptedto float on the surface of the bath contained in said tank and withinthe confines of the walls of said cylindrical member.

7. Apparatus according to claim 1 comprising a material adapted to floaton the surface of the bath contained in said tank outside the confinesof said cylindrical member.

8. Apparatus according to claim 1 wherein said vessel bottom isgenerally shaped as a portion of a sphere.

9. Apparatus according to claim 1 wherein the walls of said cylindricalmember are adapted to extend above the surface of the plating bathaccommodated in said tank.

10. Apparatus according to claim 1 wherein said tank is adapted to holda fused salt plating bath.

11. Apparatus according to claim 3 wherein said anode member is analuminum anode member.

12. Apparatus according to claim 1 wherein the curved bottom of saidpan-shaped vessel has a plurality of spaced downwardly extendingrecesses formed therein.

13. Apparatus according to claim 12 wherein a hole is formed in therecessed portions of the bottom of said pan-shaped vessel.

14. Apparatus according to claim 1 including at least one member formingprotrusions on the upper surface of the bottom of said pan-shapedvessel.

15. Apparatus according to claim 14 wherein said at least one member hasa substantially circular crosssection.

16. Apparatus according to claim 14 wherein said at least one member hasa substantially rectangular crosssection.

17. Apparatus according to claim 14 including a spirally formed memberon the upper surface of the bottom of said pan-shaped vessel formingprotrusions on said vessel.

18. Apparatus according to claim 17 comprising a plurality ofperforations formed all over the surface of said pan-shaped vesselbetween protrusions.

References Cited UNITED STATES PATENTS 1,899,679 2/1933 Fink et al.204213 843,321 2/1907 Boissier 204213 2,016,446 10/ 1935 Merkenschlager204213 FOREIGN PATENTS 546,465 3/1932 Germany 204213 JOHN H. MACK,Primary Examiner W. I. SOLOMON, Assistant Examiner US. Cl. X.R. 259-89

